1. Field of the Invention
The invention relates to wireless local area networks (WLANs), and more particularly to a mechanism for avoiding interference between Orthogonal Frequency Division Multiplexing based WLANs and other radio frequency devices where they both share the same frequency band and operate in the same environment.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) has been adopted as a mandatory part of the IEEE 802.11g draft standard for a further high-speed physical layer (PHY) extension to the IEEE 802.11b standard in the 2.4 GHz band. OFDM is well known as a highly spectral efficient transmission scheme capable of dealing with severe channel impairment encountered in a wireless environment. The basic idea of OFDM is to divide the available spectrum into several sub-channels (subcarriers). By making all sub-channels narrowband, they experience almost flat fading, which makes equalization very simple. To obtain a high spectral efficiency, the frequency responses of the sub-channels are overlapping and orthogonal. This orthogonality can be completely maintained, even though the signal passes through a time-dispersive channel, by introducing a guard interval. A guard interval is a copy of the last part of the OFDM symbol which is pre-appended to the transmitted symbol. This makes the transmitted signal periodic, which plays a decisive role in avoiding inter-symbol and inter-carrier interference.
OFDM can largely eliminate the effects of inter-symbol interference for high-speed transmission in highly dispersive channels by separating a single high speed bit stream into a multiplicity of much lower speed bit streams each modulating a different subcarrier. Hence, OFDM systems provide very reliable operation even in multipath environments that have a high degree of signal distortion. In addition, OFDM systems are able to support higher data rates than single carrier systems without incurring a huge penalty in terms of system complexity. Another significant feature of OFDM modulation is that the preamble length is shorter than other modulation schemes, for example, complimentary code keying (CCK). A shorter preamble is desirable because it results in less “overhead” on the network. Although the preamble is an absolutely essential part of the data packet, it takes up time and thereby reduces the amount of time available for data transmission. OFDM allows a shorter packet preamble to be used, thereby leaving more time for data transmission.
An 802.11g OFDM PHY offers a range of data rates from 6 up to 54 Mbps and allows for three non-overlapping channels in the 2.4 GHz Industrial, Scientific and Medical (ISM) band. In addition to 802.11g WLANs, the ISM band also hosts numerous other devices, such as Bluetooth devices, cordless telephones, and microwave ovens. Because the 2.4 GHz ISM band is unlicensed, reasonably wide, and almost globally available, it is anticipated that it will soon experience substantial crowding as low cost and high-speed wireless devices proliferate in home and enterprise environments. Although the use of the unlicensed ISM band facilitates spectrum sharing and allows for an open access to the wireless medium, it also raises serious challenges such as co-channel interference between different radio frequency (RF) systems and spectrum utilization inefficiency. Such co-channel or mutual interference may lead to significant performance degradation. The impact of interference may be even more severe, when radio transmitters of heterogeneous devices use the same band while located in close proximity. To make matters worse, the interference is likely to occur at anytime and anywhere as the majority of wireless communication devices are mobile. For example, co-channel interference occurs whenever a Bluetooth device comes within the radio coverage of an 802.11g WLAN, and vice versa. In this situation, the performance of the 802.11g WLAN is adversely affected in the presence of the Bluetooth device. Therefore, what is needed is a mechanism for an 802.11g OFDM PHY to ameliorate the effects of co-channel interference.